This invention relates to an improved laminated wood flooring for truck trailers and containers. Fiber reinforced polymer composite materials are used in conjunction with edge-glued laminated wood members to provide improved mechanical properties, moisture protection, fatigue resistance and light weight to the resulting composite boards for use in flooring of vehicular trailers.
Conventional wood flooring for over-the-road truck trailers and containers is normally manufactured with hardwoods such as oak, maple, birch, beech, etc. The green lumber used as a starting material in such manufacture is suitably dried in special drying chambers under controlled conditions. The dried lumber is then sawed into strips of rectangular cross-section and defective portions are eliminated by cross cutting the strips. During the crosscutting process, xe2x80x9chooksxe2x80x9d are formed at the ends of the lumber strips. The relatively defect-free lumber strips are coated on their vertical sides or edges with an adhesive such as urea-melamine formaldehyde or polyvinyl acetate. The uncured edge-glued lumber strips are then assembled on a conveyor by placing them side to side and behind other strips, which were previously assembled. Applying heat and edge pressure to large sections of the assembled lumber strips cures the adhesive thus forming a unitary panel. During the assembly of the lumber strips, xe2x80x9chook jointsxe2x80x9d are formed at each end of every lumber strip. These joints are simple mechanical couplings between the mating hook ends of opposing lumber strips without significant adhesive bonding at the joint itself. Often times, due to imperfect assembly, a readily visible gap is formed at the hook joints, which can be seen from the top and bottom surfaces of the finished laminated wood floor.
The cured laminated wood is cut to a desired length (up to about 60 feet) and width (about 6 to 18 inches) to form boards. The boards are then planed to a desired thickness and shiplaps and crusher beads are machined on its sides. A shiplap is a rectangular projecting lip running along the length on each side of a floor board. The crusher bead is a small semi-circular projection running along the length on each side of a board and placed over or below a lip. When the floor boards are assembled in a trailer such that the side edges of corresponding boards are squeezed together, the shiplaps of adjacent boards overlap to form a seam. The crusher beads provide spacing between adjacent boards and help in preventing buckling of the boards due to expansion on absorption of water. A wood putty is applied at the hook joints on the top and bottom surfaces of the boards to fill any resident gaps. Finally, the underside of the floor boards is coated with a polymeric substance termed as xe2x80x9cundercoatingxe2x80x9d to provide moisture protection. The finished floor boards are assembled into a kit of about eight boards for installation in trailers. Normally, a kit consists of two boards with special shiplaps so that they will fit along the road and curb sides of a trailer. The other boards may be identical in design and they are placed between the road and curb side boards. In some trailers, a metallic component such as a hat-channel may be placed between any two adjacent boards. The metallic component becomes part of the floor area. The boards adjacent to the hat-channel have shiplaps designed to mate with the flanges of the metallic component. All the boards are supported by thin-walled cross-members of I, C or hat sections, each having an upper flange, which span the width of the trailer and are spaced along the length of the trailer. Each floor board is secured to the cross-members by screws extending through the thickness of the board and the upper flanges of the cross-members.
Hardwood-based laminated wood flooring is popularly used in truck trailers since it offers many advantages. The surface characteristics of hardwoods such as high wear resistance and slip resistance are most desirable. The strength and stiffness of the flooring is important for efficient and safe transfer of the applied loads to the cross-members of the trailer. The shock resistance of wood is useful to withstand any sudden dropping of heavy cargo on the floor. Nail holding capability and ability to absorb small amounts of water, oil or grease without significantly affecting slip resistance are yet additional favorable properties of hardwood flooring.
Although the conventional wood flooring has many desirable features, it also suffers from certain disadvantages. For example, water from the roads is known to leak into trailers through the gaps of the hook joints that exist in the flooring. The reasons for the water leaks are believed to be the capillary action of the gaps and the tendency of the end grain of wood to absorb and store water. Although the undercoating is supposed to provide a barrier to the path of water, it may not properly cover larger gaps thus exposing them to moisture. Wetting and drying cycles can degrade the undercoating leading to its cracking and peeling away from the wood. Bending of the floor between two adjacent cross-members due to any applied load on the top of the floor also has a tendency to open the hook joints and enlarge the gaps.
A lift truck is often used on the trailer floor to load and unload cargo. The dynamic action of a moving lift truck placing heavy cargo on the trailer floor creates severe stress concentration in the flooring and some of the cross-members. A very large amount of the weight of the lift truck and that of the cargo is transferred to the flooring through the wheels of the front axle of the lift truck due to the momentary raising of the rear axle when the lift truck is dynamically placing a heavy cargo on the floor. The effect of repeated lift truck operation on the conventional wood floor causes considerable fatigue damage including: delamination of the edge glued lumber strips near the hook joints leading to the xe2x80x9cpop-outxe2x80x9d of the lumber strips; crack initiation and propagation in wood on the underside of the floor due to tensile stresses; and cracking of edge glue lines due to shearing, transverse bending and twisting of the floor. The combination of moisture attack and fatigue damage to the wood floor affects its performance thus necessitating its repair or replacement. In some cases, catastrophic structural failure of the trailer floor system may occur leading to the unacceptable injury to working personnel and damage to machinery.
To alleviate the above-mentioned problems, novel fiber reinforced composite wood flooring was designed, tested and refined to be an improvement over conventional wood flooring. This new composite wood flooring consists of conventional laminated wood member with an underlay of fiber reinforced plastic (FRP) bonded to the wood member. The top surface of the composite wood flooring is essentially the same as that of the conventional wood flooring. Since the FRP is impervious to the passage of water, it completely seals the bottom of the wood member and solves the problem of leaky hook joints. The fiber reinforcement improves the mechanical properties of the flooring and therefore the thickness of the laminated wood can be reduced. Thus, thinner and lighter composite wood flooring can be produced with equivalent strength when compared to thicker conventional wood flooring. Since the reinforcement provides an excellent barrier to the xe2x80x9cpop-outxe2x80x9d of lumber strips, the fatigue resistance of the composite wood flooring can be improved over that of the conventional wood flooring.
Technologists are constantly trying to find ways to improve the mechanical properties, reduce weight and improve moisture resistance of wood flooring. Fouquet, U.S. Pat. No. 5,143,418 describes the use of composite plywood panels as flooring in truck trailers. The plywood was composed of veneers of wood with a majority of the veneers oriented with the wood grain along the longitudinal direction while the remaining veneers were oriented with the wood grain along the transverse direction. The top and bottom surfaces of the plywood panels were overlaid with resin impregnated cellulose sheets for providing moisture and slip resistance. Clearly, Fouquet has not considered a floor design involving the FRP to provide higher strength and moisture protection.
Another area of related art is the use of FRP to improve the mechanical properties of structural wood members, such as beams, columns and trusses. Theakston (Canadian Agricultural Engineering, January 1965, Pages 17-19) has discussed the use of glass fibers and epoxy resin to reinforce laminated timber beams and arches. Triantafillou and Deskovic (Journal of Structural Engineering, Vol. 118, No. 5, May 1992, Pages 1270-1284) have published test results on the reinforcement of structural wood beams by adhesively bonding prestressed carbon fiber based FRP panels using epoxy adhesive. Thus the concept of reinforcing structural wood members (especially beams) with FRP has been known for several decades. Tingley, U.S. Pat. No. 5,362,545 describes the use of a resorcinol adhesive to bond certain special composite panels to glue-laminated wood beams (Glulams). The special composite panels containing aramid (Kevlar(copyright)) fiber reinforcement are abraded by sanding prior to bonding. The sanding process makes the panel xe2x80x9chair upxe2x80x9d due to Kevlar and helps to obtain improved bonding with wood. The Tingley patent teaches the utility of Kevlar in FRP panels to improve the bond strength of the FRP to wood while using a resorcinol adhesive.
The above-referenced patents and publications have not addressed the construction and related benefits of reinforced laminated wood flooring for use in truck trailers. The advantages of reinforcing the bottom side and disadvantages of reinforcing the top side of the laminated wood boards are not disclosed in these references. The publications do not discuss the remedies for the problems associated with the conventional wood flooring such as water leakage through the hook joints and fatigue damage due to lift truck traffic on the floor. There is no discussion in the publications regarding the type of reinforcements and resins that are suited for the fabrication of reinforced composite wood flooring. For example, the question of whether fiber reinforcement along the width (transverse) direction of the floor is advantageous is not addressed in these publications. A reinforced composite wood flooring construction such as that provided by the present invention which is suitable for lift truck movement and also for carrying cargo in a trailer has never before been invented.
Methods of manufacturing the reinforced composite wood boards to construct the improved flooring have never been considered before. The manufacturing process needs to be able to produce the composite wood boards at a fairly high speed to meet the demands of volume and cost effectiveness. The conventional laminated wood boards are typically manufactured at a rate of about 1500 to 2000 sq ft/hr using one set of machinery. To be competitive, composite wood boards need to be produced at a similar production rate. Since dissimilar materials with mismatch of physical properties are bonded together to manufacture composite wood boards, precaution has to be taken to ensure the flatness of the board after the bonding is completed. The adhesive used to bond the FRP and wood member should not excessively leak under the bonding pressure. Squeeze-out of adhesive can contaminate the surfaces of the substrates requiring additional clean-up operation. The bond between the FRP and wood member needs to be strong enough to resist the stress concentration around the hook joints and also be fatigue and moisture resistant.
Unlike the FRP-wood structural beams, the reinforced composite wood flooring of the present invention is designed to provide several desirable features. The composite wood floor consists of a plurality of composite wood boards. The top side of each of the composite wood boards is composed of a laminated wood member with a construction similar to that of conventional wood boards of trailer flooring. This provides a high coefficient of friction and slip resistance, thereby facilitating the safe movement of man and machine. The surface of wood can also absorb small amounts of water that may spill over it. Any oil or grease that spills on the wood surface can be wiped off and the left over slick is absorbed by the pores of wood as in the conventional wood flooring. On the contrary, even an extremely thin coating of oil on FRP can render it dangerously slippery and therefore, FRP is not laid on the top side of the composite wood board. An FRP is bonded to the bottom side of the laminated wood member to provide protection to wood from the outside environment. The FRP underlay of the composite wood flooring eliminates the need for a polymeric undercoating and for puttying of hook joints that is required in the conventional wood flooring. In fact, the need for hook joints themselves can be eliminated; that is, the lumber strips can be joined at butt ends thereof or by finger or lap and gap joints. However, the use of hook joints is preferred. The gaps in the hook joints are completely sealed by the FRP, thus preventing water leakage into the trailer through the flooring.
Preferably, the FRP is fabricated with glass fibers and an epoxy resin. Other reinforcements such as carbon and aramid fibers and other thermosetting resins such as vinyl ester, polyester, phenolic resins and the like and other thermoplastic resins such as polypropylene and polyamide resins and the like can also be used to fabricate the FRP. The glass fiber reinforcement provides an economical means of increasing the strength of the flooring in the longitudinal and transverse directions. The epoxy resin binds the glass fibers together and protects the fibers from adverse environment. The reinforcing fibers resist the pop-out of the lumber strips of composite wood floor in trailers when subjected to the fatigue loads of heavy lift trucks. The transverse reinforcement resists the splitting of the FRP and delamination of edge-glue lines in laminated wood during the application of shearing, twisting and transverse bending loads on the floor. Thus, our design of the composite wood floor provides improvements in the prevention of moisture leakage through hook joints, maintains the desirable surface characteristics of the wood on the top surface and also provides significant improvements in fatigue resistance at lower weight.
The manufacture of the composite wood boards can be accomplished by means of a suitable process wherein the fiber reinforced plastic is bonded to the surface of laminated wood member. In a wet process, the fiber reinforcement is saturated with a reactive liquid polymeric resin. The resin-wetted reinforcement is placed in contact with the surface of the laminated wood and the resin is cured under heat and pressure. A hotpress can be used to apply heat and pressure on the resin-wetted reinforcement while it is still in contact with the laminated wood. The composite wood board can also be manufactured by adapting the pultrusion process, which normally involves the pulling of resin-wetted reinforcement through a stationary heated die where the FRP is shaped and the resin is cured. The laminated wood and the resin-wetted reinforcement can be pulled together through a heated die to produce composite wood board. In the wet processes as described above, it is possible to use the same resin to bind the fibers together or form the matrix of the FRP and also to bond the fibers to the wood member.
In the above-mentioned processes, which uses liquid resins several practical problems are encountered. The simultaneous application of heat and pressure to consolidate and cure the resin-wetted reinforcement leads to squeeze-out of the resin. The applied temperature reduces the viscosity of the resin further compounding the squeeze-out of resin. The squeeze-out of the resin can occur at the sides of the layer of the wetted reinforcement or through the hook-joints of the wood member, since gaps are present at many of these joints. The composite board formed with the squeeze-out of resin needs a significant degree of subsequent machining to provide the required dimensions and shape of shiplaps along the longitudinal edges. Further, the FRP layer, which is formed under heat and bonded to a wood member under heat tends to shrink as it cools down. Due to unsymmetrical construction of the composite board with FRP bonded to one surface of a wood member and difference in the thermal expansion coefficients of wood and FRP, the composite wood board formed by heating the resin wetted reinforcements tends to warp on cooling. Typically, the composite board develops a bowed shape along the length of the board. For truck-trailers, the floor boards need to be substantially flat for easy installation and use.
In order to overcome the problems of squeeze-out of the resin and warping of the boards due to heating of the FRP for curing the resin, it is better to adhesively bond a prefabricated FRP sheet to the wood member with minimal application of heat to the FRP and wood. A suitable prefabricated FRP sheet can be bonded to laminated wood using thermosetting or thermoplastic adhesives. Thermosetting adhesives include epoxy, polyurethane, phenol-resorcinol formaldehyde, urea-melamine formaldehyde, etc. Thermoplastic adhesives include hotmelts such as ethylene vinyl acetate polymers (EVA), polyamide, etc. Experiments have shown that thermosetting liquid adhesives can be used to fabricate composite wood floor. However, squeeze-out of the adhesive through the hook joints of the wood member continues to be a problem when heat and pressure are applied. Further, application of excessive amount of heat through the FRP to quickly cure the adhesive leads to warping of the board as mentioned before. Even though the use of thermoplastic hotmelt adhesives such as EVA in bulk form or films eliminate squeeze-out, they do not lead to sufficient bond strength to resist the pop-out of the lumber strips at the hook joints. Thermoplastic hotmelt adhesives with relatively high strength can provide the required performance. After several experiments and extensive testing, reactive hotmelt adhesive has been determined to be the best choice to bond prefabricated FRP to wood members to manufacture composite wood boards. This method offers high speed of production with minimal application of heat and good bonding of the FRP to wood members to resist the opening of the hook joints under load along with little or no squeeze-out of the adhesive during production.
Reactive hotmelt adhesives behave like a typical non-reactive hotmelt adhesive during processing or bonding, but subsequently undergo chemical reactions with moisture to transform into a cross-linked thermosetting adhesive. By means of cross-linking, the molecular weight of the adhesive increases thus providing higher bond strength and hygrothermal resistance. Reactive hotmelt adhesives are solvent free solids at room temperature. They are melted by heating and then applied on the bondable surface of the substrates by means a roller coater. Typically, only one substrate needs to be coated in order to bond to another uncoated substrate. However, both substrates may be coated, if required. After coating one or both substrates with the reactive hotmelt adhesive, the substrates are joined and pressed together by a platen press or by means of rollers. The joined and pressed parts are allowed to cure for 1 to 7 days before use.
Reactive hotmelt adhesives are highly suited to the bonding of prefabricated FRP sheets to wood members for manufacturing composite flooring for trailers. The adhesive is the only component that needs to be heated to about 200xc2x0 F. to 350xc2x0 F. for the bonding process. The bonding surfaces of the wood board and FRP need to be warmed up to about 60xc2x0 F. to 200xc2x0 F., if required. Since the FRP is not substantially heated for curing its polymer, the quantity of heat required for the bonding process is minimized. This leads to the production of substantially flat boards with little or no warping after bonding. Since the reactive hotmelt is used and applied as a traditional hotmelt, it leads to high manufacturing speed. The cross-linking of the reactive hotmelt provides higher hygrothermal resistance and therefore, the composite floor can perform well under the harsh environment of the roadways. Since the reactive hotmelt has a fairly high strain to failure, it develops high peel strength, which is particularly important to reinforcing the hook joints of the wood member. When a typical brittle adhesive with a low strain to failure is used in this application, it leads to debonding of the FRP from the wood board at the regions of high stress surrounding the hook joints under an applied load on the floor board. These debonds can grow due to fatigue loads of lift truck on the floor of a trailer, leading to large-scale failure. However, the reactive hotmelt has been shown to work very well under fatigue loading in our testing and experiments. Due to a number of advantages as stated above, both in processing and performance of composite wood flooring, reactive hotmelt has been determined to be the preferred adhesive for this application. The use of a reactive hotmelt for bonding prefabricated FRP to wood members with hook joints, in order to improve its strength, stiffness, fatigue resistance and water proofing ability for use in vehicular trailer flooring has never been published before. Any suitable reactive hotmelt adhesive may be employed in the process of this invention.
The FRP sheet can be produced by pultrusion and continuous lamination processes. In the pultrusion process, the resin-wetted reinforcements are pulled under tension through a stationary heated die where the FRP is shaped and cured. In continuous lamination process, reinforcements are wetted with a polymeric resin. The wetted reinforcements are aligned in a die and then cured in an oven. Typical reinforcements used for the pultrusion process include continuous rovings, stitched, woven or knitted fabrics and continuous strand mats. In the continuous lamination process, chopped strands and chopped strand mats can be used in addition to the above-mentioned reinforcements. Preferred reinforcements for our prefabricated FRP are continuous rovings in the longitudinal direction of the FRP sheet and fabrics for transverse reinforcement. The fabrics may also be designed to provide bidirectional structural properties.
The FRP is sanded or abraded on at least one major surface in order to provide a clean surface and uniform thickness to the FRP sheet for bonding to the wood member. The wood member is prepared by planing the major surfaces. Since planing produces a lesser degree of flatness than sanding, the planed board can be sanded at least on one surface to which the FRP is eventually bonded. The planed or sanded surface of the wood board is coated with a reactive hotmelt. Preferably, both the FRP and wood member are coated on their major surfaces with the adhesive. This provides improved bonds between the FRP and wood. Alternatively, only one of the two substrates may be coated with the reactive hotmelt. By coating on both surfaces, the chances of skipping any low spots on the FRP or wood would not be a significant problem during production. After the substrates are coated, they are joined together so that the edges of the FRP are laid within a predetermined distance from the edges of the wood member. The joined parts are pressed together by a series of rollers. Simultaneously, the parts are cooled by means of forced air. The process of coating the substrates and joining them is conducted in a continuous fashion to obtain a relatively high manufacturing speed.
The present invention provides a novel process for manufacturing composite wood flooring for use in truck trailers that is subjected to lift truck loads on the top side and water spray on the bottom side. The composite wood flooring consists of a plurality of composite boards each of which is composed of a wood member with a fiber reinforced polymer adhesively bonded to the bottom side of the wood member using reactive hotmelt adhesive. Since the top side of the composite wood flooring is composed of wood, many desirable features of wood such as slip resistance, abrasion resistance and nailing capability are preserved. The underlayer of the composite wood flooring provides improved fatigue strength and moisture protection to wood. Since the mechanical properties of the reinforced composite wood flooring are superior to those of the conventional wood flooring, the thickness of the composite wood floor can be reduced leading to lower floor weight in a trailer. Alternatively, the load carrying capacity of the trailer can be increased with a suitable composite floor while having little or no increase in the weight of the trailer.
The process of this invention particularly comprises a process of manufacturing a composite wood board for use with a plurality of such composite wood boards in a floor system of a vehicular trailer or container, wherein each composite board comprises a unitary wood member with an exposed wood top surface and a bottom surface substantially bonded to a continuous planar layer of fiber reinforced polymer rendering the bottom surface substantially non-exposed, the process comprising the steps of:
providing the wood member as a side-to-side or edge bonded plurality of wood segments in an approximately planar arrangement with a substantial majority of the wood segments having shaped coupling portions at ends thereof, which ends are engaged to form several randomly placed end joints in the wood member;
machining the bonded wood segments to produce a planar wood member with two opposing major surfaces and first and second side surfaces, said wood member having a length, width, thickness and longitudinal and transverse axes;
providing a planar layer of fiber reinforced polymer having two major surfaces and a length, width, thickness and longitudinal and lateral axes, said planar layer of fiber reinforced polymer comprising a thermosetting or a thermoplastic polymer and reinforcing fibers, with at least about 70% of the fibers of said fiber reinforced polymer extending in a direction substantially parallel to the longitudinal axis of the planar layer of fiber reinforced polymer;
altering at least one major surface of the planar layer of fiber reinforced polymer by sanding or abrasion to provide a uniform thickness and clean bonding surface to the planar layer of fiber reinforced polymer;
substantially aligning the longitudinal axis of the planar layer of fiber reinforced polymer with the longitudinal axis of the wood member and substantially continuously coating one or both of the machined major surface of the wood member and the altered major surface of the planar layer of fiber reinforced polymer with a reactive hotmelt adhesive, followed by joining of the planar layer of fiber reinforced polymer to the wood member and pressing of the joined surfaces together, causing substantially no squeeze-out of the adhesive through the end joints of the wood segments and causing substantially no heat related distortion of the wood member; and
curing the adhesive under ambient conditions to bond the planar layer of fiber reinforced polymer to the wood member to produce a composite wood board;
whereby said substantially continuous planar layer of the fiber reinforced polymer bonded to the bottom surface of the wood member of the composite board resists pop-out of the wood segments at the coupling portions at the ends of the wood segments and improves one or more of the flexural modulus, strength and load carrying capacity of the composite wood board.